Influenza activity in the United States has continued to
decline since mid-January 1997. The predominant viruses have
been influenza type A (H3N2), although the proportion of
influenza B isolates has increased since the week ending January
18. This report summarizes influenza activity in the United
States from September 29, 1996, through the week ending February
15, 1997.

The proportion of patients who visited 120 U.S. sentinel
physicians for influenza-like illness (ILI) peaked at 7% from
mid-December through the first week of January and was 3% of
total visits by the week ending February 15, 1997. The
proportion of visits for ILI had remained at or below the
baseline level of 3% since the week ending January 25, 1997;
however, the proportion of ILI visits had not yet reached
baseline levels in the West South Central and Pacific regions
through the week ending February 15, 1997.

Influenza activity * has decreased since the week ending
December 28, 1996, when state and territorial epidemiologists in
38 states reported either widespread or regional activity. For
the week ending February 15, 1997, either widespread or regional
influenza activity was reported in 21 states and sporadic
activity was reported in 25 states and the District of Columbia
(Figure_1). None of the states in the East North Central region
reported regional or widespread activity for the week ending
February 15.

The proportion of deaths attributed to pneumonia and
influenza (P&I) among 122 U.S. cities exceeded the epidemic
threshold ** during the week ending December 14, 1996, and peaked
at 9.1% during the week ending January 25, 1997. Since then,
although the proportion of P&I deaths has declined, it has
remained above the epidemic threshold for 10 consecutive weeks
through the week ending February 15, 1997 (Figure_2).

From September 29, 1996, through February 15, 1997, World
Health Organization (WHO) collaborating laboratories in the
United States reported 5050 (19.1%) influenza isolates from the
total 26,430 specimens submitted for respiratory virus testing:
4714 (93.4%) were type A, and 336 (6.7%) were type B. All 1866
influenza A isolates subtyped have been A(H3N2) viruses; thus
far, no A(H1N1) viruses have been reported in the United States
during the 1996-97 influenza season. From September 29, 1996,
through December 28, 1996, a total of 38 (1.4%) of 2811
influenza isolates were type B. Although the total number of
influenza viruses isolated has declined since then, the
proportion of influenza B isolates has increased. During January
26-February 15, a total of 166 (42.5%) of the 391 reported
influenza isolates were type B. At least one type B isolate has
been reported from each region.

Reported by: Participating state and territorial epidemiologists
and state public health laboratory directors. World Health
Organization collaborating laboratories. Sentinel Physicians
Influenza Surveillance System. Influenza Br and WHO
Collaborating Center for Surveillance, Epidemiology, and Control
of Influenza, Div of Viral and Rickettsial Diseases, National
Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: All four components of the influenza
surveillance system indicate that influenza activity is
declining in the United States. However, as of February 15, some
states continued reporting widespread activity. Although 93% of
influenza isolates for this season have been type A, an
increasing proportion of influenza viruses isolated by WHO
collaborating laboratories since January 1997 have been
influenza type B viruses.

Rapid antigen testing of nasopharyngeal swab specimens for
influenza A is commercially available in many areas. The timely
diagnosis of influenza A can be useful in inpatient and clinic
settings to guide the selection of antiviral drugs (amantadine
or rimantadine) for prophylaxis or treatment of persons at
high-risk for influenza A-related complications. These drugs are
70%-90% effective in preventing influenza A infections and can
reduce the severity and duration of symptoms from influenza A
when administered within 48 hours of illness onset. However,
they are not effective against influenza type B viruses.

Early recognition of influenza A outbreaks is especially
important in institutions that provide care for elderly persons
because infection can spread rapidly and the impact of influenza
A can be particularly severe in these settings. Administration
of amantadine or rimantadine early in the course of an influenza
A outbreak can control further spread of infection. Chronic-care
facilities should know before an outbreak occurs which
laboratories in their area perform influenza A rapid antigen
testing (1-3).

Influenza surveillance data collected by CDC is updated
weekly throughout the influenza season. Information is available
through the CDC voice information system, telephone (404)
332-4551, or the fax information system, telephone (404)
332-4565, by requesting document number 361100.

ACIP. Prevention and control of influenza: recommendations of
the Advisory Committee on Immunization Practices (ACIP). MMWR
1996;45(no. RR-5).

Levels of activity are 1) no activity; 2) sporadic --
sporadically
occurring ILI or culture-confirmed influenza with no outbreaks
detected; 3) regional -- outbreaks of ILI or culture-confirmed
influenza in counties with a combined population of less than 50%
of
the state's total population; and 4) widespread -- outbreaks of ILI
or culture-confirmed influenza in counties with a combined
population
of greater than or equal to 50% of the state's total population.
** The epidemic threshold is 1.645 standard deviations above the
seasonal baseline. The expected seasonal baseline is projected
using a robust regression procedure in which a periodic
regression model is applied to observed percentages of deaths
from P&I since 1983.

DisclaimerAll MMWR HTML versions of articles are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the electronic PDF version and/or the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.

**Questions or messages regarding errors in formatting should be addressed to mmwrq@cdc.gov.